Chemical mechanical polishing (CMP) is one technique which has been conventionally used for semiconductor wafer planarization. For example, see U.S. Pat. No. 5,099,614, issued in March in 1992 to Riarai et al.; U.S. Pat. No. 5,329,732 issued July 1994 to Karlsrud et al., and U.S. Pat. No. 5,498,199 issued March 1966 to Karlsrud et al. A typical CMP apparatus suitable for planarizing a semiconductor surface generally includes a wafer carrier configured to support, guide, and apply pressure to a wafer during the polishing process, a polishing compound such as a slurry to assist in wafer surface material removal, and a polishing surface such as a polishing pad. In addition, the polishing apparatus may include an integrated wafer cleaning system and/or an automated load/unload station to facilitate automatic processing of the wafers.
A wafer surface is generally polished by moving the wafer to be polished relative to the polishing surface in the presence of a polishing compound. In particular, the wafer is placed in a carrier such that the surface to be polished is placed in contact with the polishing surface, and the polishing surface and the wafer are moved relative to each other while slurry is supplied to the polishing surface.
Furthermore, CMP is often applied when forming microelectronic devices to provide a substantially smooth, planar surface suitable for subsequent fabrication processes such as photoresist coating and pattern definition. For example, a conductive feature such as a metal line, conductive plug, or the like may be formed on a wafer surface by forming trench lines and vias on the wafer surface, depositing conductive material over the wafer surface and into the trenches and vias, and removing the conductive material on the wafer surface using a CMP process, leaving the vias and trenches filled with conductive material. The conductive features often include a barrier material to reduce unwanted conductive material diffusion and to promote adhesion between the conductive material and any adjacent layer in the circuit.
Aluminum is often used to form conductive features because its characteristics are compatible with conventional deposition (e.g. chemical vapor deposition) and etch (e.g., reactive ion etch) techniques. Although using aluminum to form conductive features is adequate in some cases, forming aluminum conductive features becomes increasingly problematic as the size of the conductive feature decreases. In particular, as the conductive feature decreases in size, the current density through the feature generally increases, and thus the feature becomes increasingly susceptible to electromigration, i.e., the mass transport of metal due to the current flow. Electromigration may cause short circuits where the metal accumulates, open circuits where the metal has been depleted, and/or other circuit failures. Similarly, increased conductive feature resistance may cause unwanted device problems such as access power consumption and heat generation.
Recently, techniques utilizing copper to form conductive features have been developed because copper is less susceptible to electromigration and exhibits a lower resistivity than aluminum. Since copper does not readily form volatile or soluble compounds, the copper conductive features are often formed using a damascene process. More particularly, the copper conductive features are formed by creating a via within an insulating material, depositing a barrier layer onto the insulating material surface and into the via, depositing a seed layer of copper into the barrier layer, electrodepositing a copper layer onto the seed layer to fill the via, and removing any excess barrier metal and copper from the insulating material surface using chemical and mechanical polishing. During the electrodeposition process, additives such as leveling agents may be added to the plating bath to reduce the formation of voids within the conductive features.
As stated previously, a CMP machine typically includes a wafer carrier configured to hold, rotate, and transport a wafer during the process of polishing or planarizing the wafer. During the planarizing operation, a pressure applying element (e.g., a rigid plate, a bladder assembly, or the like) that may be an integral part of the wafer carrier applies pressure such that the wafer engages a polishing surface with a desired amount of force. The carrier and the polishing surface are rotated, typically at different rotational velocities, to cause relative lateral motion between the polishing surface and the wafer and to promote uniformed planarization. The polishing surface generally comprises a horizontal polishing pad that may be formed of various materials such as blown polyurethane and are available commercially from, for example, Rodel Inc. located in Phoenix, Ariz. Abrasive slurry may also be applied to the polishing surface which acts to chemically weaken the molecular bonds at the wafer surface so that the mechanical action of the polishing pad and slurry abrasive can remove the undesirable material from the wafer surface.
Unfortunately, the CMP process tends to leave stresses in the workpiece leading to subsequent cracking and shorting between metal layers. Furthermore, the CMP process may result in sheering or crushing of fragile layers. The CMP process also has a tendency to cause dishing in the center of wide metal features, such as trenches and vias, oxide erosion between metal features, and dielectric oxide loss.
For example, a conventional manufacturing process for a dual damascene structure includes a step of polishing a low dielectric constant material (i.e., k<2.6) to create a planar surface in preparation of a copper layer deposition. Planarizing the low dielectric constant material is necessary because the low dielectric constant material is deposited atop an underlying metal surface that generally has undulations resulting from a prior CMP polishing step, and the low dielectric constant material conforms to the underlying surface topography.
After the low dielectric constant material is polished, a copper layer is deposited into a via extending therethrough, and copper also naturally forms as a layer atop the low dielectric constant material. Using a CMP process, the copper layer is removed, leaving the low dielectric constant material and an exposed copper surface inside the via. The exposed copper typically has a dished surface following the CMP process. Consequently, the low dielectric constant material typically must be subjected to additional polishing after the copper CMP process. Polishing the low dielectric constant material is problematic. The dielectric material is typically both porous and fragile, and as a result is vulnerable to losing dielectric properties if water, water vapor, or other relatively high dielectric constant materials are entrained or adsorbed to the porous dielectric surface. Since many polishing slurries are aqueous or have a relatively high dielectric constant, it is difficult to polish without deteriorating the low dielectric constant material's dielectric properties. One way to avoid damaging the low dielectric constant material is to deposit a capping layer above the low dielectric constant material. The capping layer enables the dielectric stack to withstand friction at sustained temperatures as high as 400° C. A thin silicon carbide (SiC) material containing hydrogen is particularly effective as the capping layer. SiC has a dielectric constant between 4.5 and 7.5. Another way to avoid overstressing the material is to polish the exposed copper surface within the via at very low pad pressures and moderate sheer rates. However, both of these preventive measures increase polish time and decrease tool throughput.
One alternative to CMP for minimizing surface topography for a low dielectric constant layer is the use of an abrasive-free polish (AFP) slurry to polish the underlying copper layer surface before depositing a dielectric material thereon. Polishing copper using an AFP slurry is a particularly effective way to minimize dishing, and consequently minimizes or eliminates corrective dielectric polishing following the copper polishing. An AFP slurry effectively planarizes the copper surface when the topography is already relatively flat and if there is not a large copper overburden or field area thickness. However, not all copper deposition processes result in a suitably flat copper surface or a sufficiently thin copper layer for the AFP slurry to be effective or efficient.
Electrochemical planarization, also known as electropolishing, is another attractive alternative to CMP because it does not impart significant mechanical stresses to the workpiece, and consequently does not significantly reduce the integrity of the devices. Furthermore, electrochemical planarization is less likely to cause metal dishing, oxide erosion, and oxide loss of the dielectric layer.
Electrochemical planarization is based on electroetching and electrochemical machining, that is, the removal of a thin layer of metal from a substrate through the action of an electrical solution and electricity. For example, if two electrodes, an anode and a cathode are immersed in a liquid electrolyte and are wired to permit a potential difference between the electrodes, metal atoms in the anode are ionized by the electricity and go into the solution as ions. Depending on the chemistry of the metals and salt, the metal ions from the anode tend to either plate the cathodes, fall out as precipitate, or remain in solution. Unfortunately, using conventional electrochemical planarization techniques, etching selectivity is reduced in areas on the wafer having varying high or low topographies, and uniform planarization is not achieved. The same is largely true when a polish pad is used in conjunction with an electrochemical planarization process, in which case the process is referred to as electrochemical mechanical planarization, or ECMP.
Accordingly, it is desirable to overcome some of the difficulties associated with forming a damascene structure, particularly one that includes an interlayer dielectric structure with multiple dielectric layers. Particularly, it is desirable to overcome problems related to surface topography on a metallization or dielectric layer due to uneven layer deposition, and to overcome associated difficulties in planarizing low dielectric constant materials formed around or above the metallization layer. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.